Ever since the automobile was invented people have been working to make it more efficient. Obtaining the maximum energy out of the fuel that propels a vehicle is an age old problem which has been tackled in a number of manners. Recently, people have been improving vehicles' efficiencies by recovering the excess kinetic energy that is transformed into heat when a vehicle brakes or decelerates. Modern systems rely upon computer control to detect operating states of the vehicle, such as using the output from an engine control unit. Other systems are directed towards making hybrid vehicles—that is vehicles with both an electric motor and an on-board fueled electricity generator—more efficient by using the electric motor to recover and convert kinetic energy into stored electrical energy.
A major disadvantage of the modern systems is that they require modern automobiles. Computer controlled engines have been around for several years, but there are a great many automobiles being driven that do not have complex electronic controllers in them. These older automobiles are not able to take advantage of the fuel savings modern technology can offer.
Another drawback of many of the modern systems is that they are expensive and complex. Greater reliance upon computer control to operate a system increases the complexity of the system as well as the likelihood that something will malfunction.